1. Field of the Invention
This invention relates to the supply of direct current to a telephone subscriber loop, and in particular to apparatus for providing a controlled current to the subscriber loop from a floating current source.
2. Background Description
The problem of supplying operating power from a central office to a subscriber telephone set is a continuing one. The arrangements for doing so are variously described as line feed circuits and telephone subscriber line circuits. For the standard telephone circuit the technique for supplying the operating current to the subscriber loop consisted essentially of two resistors which are designated as line feed resistors and these are in series with the central office battery. The voltage amplitude of the battery is in the range of 48 to 60 volts and the line feed resistors typically have resistance values ranging from 200 to 800 ohms depending upon the particular telephone exchange. These line feed resistors are selected at the central office to have a value which would produce the proper line current for the loop resistance characteristic expected by the telephone instrument to which it is to be connected.
Another common technique is by the use of a transformer, the primary of which is connected to the voice frequency circuit. The secondary consists of two windings, the two secondary windings being joined by a blocking capacitor, which provides a low AC impedance for voice frequencies and a high DC impedance. The battery voltage is applied to the one and other terminal ends of the capacitor. One problem with this approach is that loop supervision, i.e., on-hook, off-hook and dial pulsing information could not be detected by using the battery feed circuit. Thus, separate detection circuits are required. These traditional circuits are not energy efficient, because of the excessive loss in the line feed resistors.
More recently, floating battery feed circuits, which are more energy efficient, have been devised. One such circuit is disclosed in the U.S. Pat. No. 4,056,691 entitled, "Telephone Subscriber Line Circuit", L. Freimanis et. al., granted Nov. 1, 1977. In the referenced patent it is clear that the battery feed current is coupled to the telephone insturment from a source of positive and negative potential that "floats" (has a very high impedance) relative to earth ground. The main advantage of this method is that of very good longitudinal balance, which provides good immunity to AC induction. As shown in FIG. 3 of the referenced patent a transformer is included which has, in addition to the normal primary and secondary windings, a plurality of sense windings which are used to ensure a constant current supply to the subscriber loop when the battery feed circuit is turned on. Further, a number of external control circuits are required. For example, a scan flip-flop is used for status checks.
A technique by which a regulated loop current may be applied to the subscriber line is disclosed in U.S. Pat. No. 4,315,106, entitled "Apparatus For Regulating Current Supplied To A Telephone Line Signal Of The Type Employed In Digital Telephone Systems", inventor R. C. W. Chea, Jr., granted Feb. 9, 1982. In order to reduce the power consumption and thereby improve the energy efficiency, the circuit disclosed replaces the battery and series line feed resistors with the power supply circuit in which the line voltage is monitored and this information is used to control the actual supply voltage via the line feed resistors to the subscriber loop. By using this control technique the size of the line feed resistors can be reduced so that power dissipation is also reduced.